Art of cracking hydrocarbons



Feb. 28, 1928.

E. W. ISOM ART OF CRACKING HYDROGARBONS Filed May 15. 1925 ill INVENTOR L 0 14 07] 144 f Jam BY M Y'W ATTORNEYS sure distillate, therefrom.

larly, the invention relates to improvements Patented Feb. 28, 1928.

UNITED STATES PATENT OFFICE.

EDWARD W. ISOM, OF iLOCUST VALLEY, NEW YORK, ASSIGN OR '10 SINCLA IR. REFINING COMPANY, OENEW YORK, N. 'Y., .A CORPORATION OF MAINE.

ART OF CRACKING HYDROCARBONS.

Application and m 15, 1925. swarm. 30,404.

This invention relates to improvements in the cracking of heavier hydrocarbon oils, such as gas oil or kerosene, to produce lighter hydrocarbon oils, such as gasoline or pres- More particuin the operation of externally heated single pass tubular oil cracking still and in the operation of heating furnaces employed in conjunction with cracking stills of this type.

This application is in part a continuation of my prior application filed August 28,

1922, Serial No. 584,711.

In a single pass'tubular oil cracking still,

the oil is passed but once through the' oilf heating tubes and the entire heat input to the oil is effected in the single passage of the oil through the heating tubes, the oil supplied to the still'being brought from its initial ten'lpe-rature to the cracking temperature employed in the single passage of the voil through the heating tubes. For efliciency of operation and economy of equipment,

it is desirable to transfer the maximum" employ heating gases at temperatures which do not tend to cause overheating and WlllCll do not impose severe strains on the still equipment. Hitherto, these apparently contrary factors have prevented development of maximum heating efliciency in externallyhealed single passtubular oil cracking stills. Moreover, with the usual fuels, e. g. gas or tucloil or coal, temperatures promoting cllicicntcombustion are substantially higher than the temperatures at which it is de sirable to have the heating gases contact with the oil heating tubes of the cracking still. Here also, previous proposals toreduce the temperature of the heating gases passing over the oil heating tubes, e. g. dilution of the hot products of combustion with cold air or carrying out the combustion with a large excess of air, have involved a decrease in heating efiicicncy.

()ne of the objects of the presentinvem tion is to provide an improved method of heating externally heated single pass tubular oil cracking stills'in which high average temperature differentials are maintained between the heating gases and the oil circulating through the heating tubes-in a single and also in which the heating gases passing over the -oil 'h eating tubes are tempered wh le mamtaimng a high temperature of combustion, without decreasing but rather increasing the efficiency of the heating operation.

Acc'or ding to the present invention, the 011 heating tubes of the cracking still are arranged in a heating furnace through which .hot products of combustion are circulated from a fire-box in which fuel is burned for supplying heat to the cracking operation,

the o1 undergoing cracking is circulated through-the .oil heating tubes in a single pass, and part of the heating gases which have passedover the oil heating tubes in the heating furnace are recirculated and admixed with-the fresh hot products of com- ,bustion, the resulting mixture of heating the oil heating tubes gases being passed over in the furnace.

' The oil circulated through .the heating tubes in a single pass may be discharged into a drum or shell or other suitable receptacle or reservoir in which the cracking operation is continued or in which another cracking operation is carried out, or the crackingoperation may be substantially completed inthe oil heating tubes in the furnace. The

vaporsof the cracked oil may also be subjected to a refluxing operation and the reflux may with advantage be circulated in a single pass through the .oil heating tubes in admixture with the fresh oil. Circulation of the oil through the heating tubes is advantageously maintained in a generally countercurrent direction to the flow of heating gases over the heating tubes. The combustion of the fuel employed may be car ried out at as high a temperature as is desired for eilicient combustion of the fuel, and ,it is also advantageous to efl'ect the combustion without substantial'excess of-air to further improve the fuel economy.

The invention will be further described in connection with the accompanying drawings which illustrate in a diagrammatic and conventional manner, in elevation and partly in section, an apparatus adapted for the practice of the process of the invention, but it' is intended and will be understood that this. further description and illustration are for the purpose of exemplification and that the invention is not limited thereto. In the drawings, a single pass externally heated tubular cracking still is shown discharging into a shell still or reservoir from which the vapors of the cracked oil are taken off. The single pass cracking still comprisesa furnace 1 in which a battery of heating tubes 2 are arranged, oil being supplied to the heating tubes through connection 3 and after passing in a single pass therethrough being discharged through connection 4. The

heating furnace 1 is provided with a firebox 5 and a stack flue 6, heating gases being circulated from the fire-box through the heating furnace over the oil heating tubes therein to the stack flue. The tire-box 5 is provided with means for burning the fuel employed to supply the heat used in the cracking operation. A burner for gaseous or liquid fuel is shown at 6. Before the hot products of combustion from the tire-box 5 come in contact with the oil heating tubes 2 in the furnace 1, they are admixed with a part of the heating gases which have passed over the oil heating tubes and which are returned through connection 7 The connection 7 is provided with an inlet 8 adjacent the exit to the stack flue from the furnace and with an outlet 9 through which the heating gases withdrawn from near the out-. let of the furnace. are returned and admixed with the hot products of combustion. The return connect-ion 7 includes a blower 10 which may be operated by a steam jetll or other suitable means for maintaining circulation of the returned heating gases. In place of the blower shown, a fan or other suitable forcing means may be employed. The fresh hot products of combustion are thus diluted with the recirculated cooler gases which have passed over the oil heating tubes and are thereby tempered before the resulting mixture of heating gases is passed over the oil heating tubes. vAt the same time, the temperature of combustion in the fire-box 6 may be regulated more or less independently of the temperature conditions which it is desired to maintain in the heating furnace proper.

In the apparatus illustrated, the oil discharged from the single pass cracking still through connection 4 is introduced into the reservoir 12 through pipe 13'. Vapors from the shell still escape through connection 14 to dephlegmators 15 and 16. Vapors escaping from dephlegmator 16 through connection 17 are'discharged to a condenser (not shown) in which the vapors of the cracked oil are-condensed. Pressure in the cracking system maybe maintained andregulated by a valve 18 arranged between the shell still and the dephlegmators, or bya valve 19 arranged between the dephlegmators and the condenser, or by suitable valves arranged through connection 25.

emma beyond the condenser. A connection 20 is shown for withdrawing tar and residue frmn the 'shell still 12. The unvaporized residue .of the oil discharged into the reservoir from subjected to a cracking operation therein by the heat of the products discharged from the singlepass cracking still. Oil so supplied to the reservoir may be pumped from a receptacle 21 through the dephlegmators l5 and 16 in indirect heat exchanging relation with the va'pors therein for cooling and dephlegmating the vapors and thence into the reservoir through connection 22 and the pipe 23. Reflux from the dephlegmators l5 and 16 may be discharged to the receptacle 24, or the heavier reflux from the dephlegmator 15 may be returned to the reservoir This reflux may with advantage be pumped in a single pass through the heating tubes 2 in the single pass cracking still by means of pump 26 together with fresh oil supplied to the single pass cracking still from the receptacle 27. Reflux may be so returned while still warm. The reservoir may be heated by means of furnace 28 in addition ,to the heat supplied internally! by' the hot products discharged from the single pass cracking still, or it maybe heated internally only by'the hot products discharged from the single; pass cracking v still. The reservoir, articularly where it is heated internally on y by the hot products discharged from the single pass cracking still, and the connections between the single pass still and reservoir may witlradvantage be lagged to prevent heat loss.

In the heating furnace of the single pass cracking still, the temperature of the gas mixture first contacted with the oil heating tubes is decreased and the temperature of the heating gases escaping from the heating furnace are increased as the rate or recirculation of heating gases through connection 7 is increased. The temperature conditions within the heatingfurnace may thus be conveniently controlled gby-regulating the rate of recirculation of heating gases. This control, moreover, is largely independentl of the temperatures maintained in the fireox 5, so that in the fire-box temperatures 'most'suitable for efficient combustion of the fuel burned can be employed. In operation, for example, the temperature of the oil discharged from the single pass cracking still may vary from about 700 to 1200 1*. At the same time, whilemaintaining a combustion temperature of as much as 2100 to 2700 F. or more, the temperature of the heating gases, including the Ill) lib

and admixing them with the fresh hot prodnets of combustion. The temperature of the heating gases leaving the heating furnace through the stack fine 6 may approximate 450 to 700 F. or more in such operations.

' It will be understood that the foregoing temperatures are given as illustrations of the conditions which may be maintained in the heating furnace in carrying out the process of the invention and that the inventionis not limited thereto. These temperatures described as illustrating the invention were, maintained while using gas, or

gas and fuel oil, as fuel in operations in which charging stock of a kerosene character, or somewhat heavier charging stock of a gas oil character, or including in addition reflux constituents, were'circulated in a single pass through the oil heating tubes in the heating furnace. v

The recirculated heating. gases may be admixed with the hot productsof combustion in the fire-box, or they may be admixed with the hot products of combustion before or as they pass over the single pass oil heating tubes. The heating gases in either case are thus tempered in an advantageous way' with improvement of the heatingoperation while an amount of fuel is saved corresponding approximately to the amount of heat which would be necessary to raise a cold diluent, e. g. cold air, to the temperature of the recirculated heating gases in ad dition to the amount saved because of the improved efficiency of the heating operation itself. Where part of the heating gases which have passed over the oil heating tubes are returned directly to the fire-box the temperatures in the fire-box may be somewhat reduced and all or part of the recirculated heating gases may thus be introduced into the fire-box proper, for example throu 11 ports in the walls of the fire-box,to assist in protecting the fire-box construction. The temperature of the actual combustion may nevertheless be maintained at a high value, but, even where the temperature of combustion is somewhat reduced, the economies in fuel and the im 'rovement in eflicienc which are effected y the relatively high temperature at which the diluent, i. e. the

recirculated heating gases, are introduced,

and the increase in volume of the heating gases circulating over the single pass oil heating tubes are secured.

The present invention has several important advantages. It will be a parent that in carrying out the invention t 0 fuel economy is improved in several ways. By

'maintained, without subjecting the heating providing for tempering of the heating gases passing over the oil heating tubes in the single pass cracking still by recirculation of part of the heating gases which have passed over the oil heating tubes,,high combustion temperatures at which efficient combustion of the fuel is secured may be employed Without detriment to the cracking operation carried out in the single pass heating tubes and without imposing severe strains on the still equipment, A high average temperature differential between the heating gases and the oil circulating in a single pass through the heating tubes may be tubes to heating gases at excessive temperatures, further promoting the effectiveness of the heat transfer. Likewise, the recirculation of part of the heating gases which have passed over the heating tubes increases the volume and hence the velocity of flow of the heating gases, further improving the efliciency of the heat transfer. As compared to an operation in which excess cold air is employed to dilute the products of combustion, the heat necessary to raise the temperature of the cold'air to.tl1e,temperature at which the heating gases are discharged from the heating furnace is saved. While in carrying out the invention the tempera ture at which the heating gases escape is increased, the amount of heating gases escaping may be reduced so that the amount of heat escaping with the heating gases which are not recycled may not be increased but may evenbe somewhat decreased. All of these improvements assist in reducing the amount of'fuel re uired, Also, b improving the efficiency 0 the heat trans er-and by maintaining a high average temperature 105 differential throughout the heatin operation between the heating gases an the oil, circulating in a single pass through the heating tubes, the amount of heat which can be transferred to the oil in a given distance 110 of oil travel is substantially increased although excessively high temperatures'are at the same time avoided. Due to the improved uniformity of the heating opera tion, higher temperatures may also be employed in the heating furnace where desired. Another advantage of the invention is the improved control of the heating opertion which it affords.

In referring herein to a single pass crackno heating tubes, but it is to be distinguished no from operations in which the freshoil itself of oil once through an elongated heating conduit, burning fuel and circulating the c hot products of combustion over the oil heating tubes in a direction generally countercurrent to the flow of oil therethrough, whereby the temperature of the heating gases last contacting with the heating conduit is reduced to below the temperature at which the hot oil products are discharged from said heating conduit, recirculating part of the said heating gases which have passed over theoil heating tubes and admixing them with the fresh hot products oft-combustion and circulating the resulting mixture of heating gases over the oil heat- .ing tubes.

2. In the operation of externally heated single pass tubular oil cracking stills, the improvement comprising forcing a stream of oil once through an elongated heating conduit, burning fuel without any substantial excess air and circulating the hot products of combustion over the oil heating tubes in a direction generally countercurrent to the flow of" oil .therethrough, whereby the temperature of the heating gases last contacting with the heating conduit is reduced to below the temperature at which the hot oil products are discharged from said heating' conduit, recirculating part of the said heating gases which have passed over the oil heating tubes and admixing them with the fresh hot products of combustion and circulating the resulting mixture'of heating gases over the oil heating tubes.

3. In the operation of externally heated single pass tubular oil cracking stills, the improvement comprising forcing a stream of oil once through an elongated heating conduit, burning fuel and circulating the hot products of combustion over the. oil heating tubes in a direction generally coun tercurrent to the flow of oil therethrough, whereby *th'e'temperature of the heating gases last contacting .with the heating con duit is reduced to-below-the temperature at which the hot'oil products are discharged from said heating conduit, recirculating part of the said heating gases which have passed over the oil heating tubes-and admixing them with the'fresh hot products of combustion and circulating the resulting mixture of heating gases over the oil heat-'- ing tubes, subjecting the vapors of the cracked oil to a refluxing operation, and circulating reflux therefrom through the oil heating tubes in a single pass in admixture with the fresh oil.

In testimony whereof I afiix my signature.

EDWARD W. ISOM. 

